This invention relates to a gas absorption refrigeration system that can operate at temperatures as low as -40.degree. C. and, more particularly, to an improved gas absorber for use in such a system for causing a refrigerant gas such as ammonia to be absorbed into an absorbent salt solution.
Mechanical compressor systems have long dominated the field of refrigeration, but the ozone-depleting characteristics of chloro-fluoro-carbons are a serious problem. Moreover, mechanical vapor compression systems are found to have little thermodynamic superiority over absorption systems as a result of the need to convert thermal energy into mechanical energy at a low temperature for the compression system. Consequently, there is currently a renewed interest in absorption refrigeration systems.
Various aspects of this technology have been discussed by Carlos Alberto Infante Ferreira in his thesis entitled "Vertical Tubular Absorbers for Ammonia-Salt Absorption Refrigeration" published by The Technische Wetenschappen aan de Technische Hogeschool, Delft, Holland; Mar. 26, 1985. In order to improve the performance of an absorption refrigeration system for given working conditions, Ferreira considered firstly to identify favorable absorbent-refrigerant mixtures from a thermodynamic standpoint, secondly to improve the efficiency of system components, and thirdly to effect system modifications to the standard refrigeration cycle. Ammonia has been considered as one of the best refrigerants, and many salt solutions have been investigated. Ferreira considered sodium thiocyanate (NaSCN) solution to be the best but saw a definite advantage in the NH.sub.3 --NaSCN--NaI system because possible salting out effects are of concern only at lower temperatures and concentrations. Although significant efforts have been expended to find preferable salt solutions, however, the identification of new salt solutions has been only partially successful, and it is therefore an object of the present invention to provide an improved system for carrying out a refrigeration cycles to expand the capabilities of presently known absorption refrigeration systems.
The temperature lift capability of a salt solution as applied to refrigeration systems is typically limited by its solubility characteristics. As a 50--50 mol % NaI--NaSCN ammonia solution is heated, for example, ammonia is driven off and when the salt concentration is approximately 75% (at equilibrium temperatures and pressures), the solution begins to crystallize due to insufficient ammonia and solubility limits. The higher the pressure, the higher the temperature must be to drive off ammonia to achieve the 75% level. If the temperature is lower than the equilibrium temperature, the vapor pressure will be lower than the equilibrium pressure. If the temperature is lowered further, the solution will crystallize at atmospheric pressure. In other words, beyond the solubility field of a solution, which is dependent on temperature and pressure, crystallization occurs, leading to failure of the refrigeration system.
A critical component of all absorption refrigeration systems is an absorber for causing the refrigerant to be absorbed by a refrigerant solution. Absorption of ammonia is an exothermic reaction, which requires that the absorbing medium be cooled effectively because, if the temperature rises, the equilibrium condition of the reaction shifts such that the pressure inside the vessel may potentially rise. Two types of absorbers which seem to be in general service are a vertical water-cooled, shell-and-tube absorber-condenser used in conjunction with a cooling tower water circulation system and an air-cooled absorber condenser. Water-cooled absorbers which derive cooling water from cooling towers operate at relatively high temperature differences between the ambient temperature and the absorber outlet and require larger amounts of water flow, if the temperature gain in the water is to remain small, as is required to maintain the lowest possible absorber pressure necessary for low temperature operation. The cooling pumps require much power and the piping can become very expensive, if the cooling tower cannot be located near the absorber. In addition, fans are required to operate the cooling tower. Air-cooled absorbers, on the other hand, require very large heat transfer surfaces and also consume a great deal of electrical power to drive the cooling fans.
It is therefore a general object of the present invention to provide an efficient absorption refrigeration system.
It is a more specific object of the present invention to provide an absorption refrigeration system with a refrigeration cycle capable of operating at temperatures as low as about -40.degree. C. with an improved margin of safety.
It is another object of the present invention to provide an efficient, cost-saving and relatively compact gas absorber for such an absorption refrigeration system so as to increase the margin of safety to crystallization along the process path of the refrigeration system.